1. Field of the Invention
The present invention relates to image forming apparatuses such as a printer and a copying machine and a method of controlling the image forming apparatuses.
2. Description of the Prior Art
Examples of a printer conventionally known include one comprising a developer supplying roller 1 for supplying to a predetermined print position a developer T charged to predetermined polarity, for example, negatively charged, a print head 6 provided in the print position and having an electrode pair, which is provided with a developer passing hole 2, comprising an upper first electrode 3 and a lower second electrode 4 connecting with each other, and a paper feeding roller 7 for feeding paper P to the print position, and so adapted that the developer passing hole 2 is opened or closed in a potential manner by the control of the potentials of the first electrode 3 and the second electrode 4 on the basis of print dot data sent in synchronization with a predetermined print clock signal.
Although in the print head 6, a plurality of electrode pairs each comprising a first electrode 3 and a second electrode 4 are actually arranged in a matrix shape with an insulating substrate 5 being interposed therebetween and respectively provided with a plurality of developer passing holes 2, only one electrode pair comprising a first electrode 3 and a second electrode 4 is illustrated for convenience.
The first electrode 3 is grounded. A predetermined positive voltage V3 is applied to the paper feeding roller 7. Consider a case where the print dot data is printed. In this case, when V1 is taken as the potential of the first electrode 3 and V2 is taken as the potential of the second electrode 4, the potential V2 of the second electrode 4 is so controlled that V1&lt;V2&lt;V3. Consequently, a developer T passes through the developer passing hole 2 in the print head 6 from the developer supplying roller 1 and adheres to the paper P by an electric field formed by the first and second electrodes 3 and 4 as well as the paper feeding roller 7.
Consider a case where the print dot data is unprinted. In this case, the potential V2 of the second electrode 4 is so controlled that V1&gt;V2. Consequently, the developer T is prevented from passing through the developer passing hole 2 in the print head 6 by an electric field formed by the first electrode 3 and the second electrode 4.
In such a printer, the developer T is not sufficiently supplied to the developer passing hole 2. Accordingly, such an electric field that the developer T is attracted from the developer supplying roller 1 to the first electrode 3 has been conventionally always generated between the developer supplying roller 1 and the first electrode 3. More specifically, assuming that the developer T is negatively charged, the positive pole of a DC power supply 12 is connected to the first electrode 3 and the negative pole of the DC power supply 12 is connected to the developer supplying roller 1, as shown in FIG. 3.
If such an electric field that the developer T is attracted from the developer supplying roller 1 to the first electrode 3 is always generated between the developer supplying roller 1 and the first electrode 3, the developer T is sufficiently supplied to the developer passing hole 2. Since the developer T is also supplied to developer passing holes 2 other than the developer passing hole 2 corresponding to the print dot data which is printed, however, the developer passing holes 2 are clogged with the developer T.